Large trailer hitch help, please

[FEF]

All,

I've got my fuel tanks mounted, and I'm to the point where I'm giving more serious thought to my choice of trailer. As there are engineer types here, and you guys don't laugh at the idea of trailers, I thought I might see what you all thought.

The question is... What forces change, when you replace a fifth wheel pin box, with a pin box that has a pintle ring on the front?

It's easy to see how the center line of the vertical load changes. There will be a cantilever action, with the front of the pin box as the fulcrum. I can easily account for that.

The pin box will want to rotate, as I drag the trailer left and right. That shouldn't be hard to account for, too. That happens with extended pin boxes.

All the other forces should be similar, right? As I've seen short pin boxes, and extended pin boxes, the pintle hitch on a short box should not be any worse then the fifth wheel pin on a long pin box.

In fact, what I'm thinking about doing is having a pin box made that will have the fifth wheel pin on bottom, and a pintle ring on front. Setting up the truck is easy, and not an issue.

Any thoughts?

Thanks for the help,

 

[ntsqd]

I've not done an FBD or anything else, so this is all from my brain's FEA package.

With the 5th pin there is very little torque possible on the box (friction of the plates only), so it's torsional rigidity isn't a high design priority. Though it can have bending moments, fore/aft from towing loads, side to side from a change in surface angle.
No matter where you put the lunette it will introduce a torsion at some point in use, though some locations are better than others. How much that matters or how large the torque is I've no idea.

Does a Gooseneck not have enough possible misalignment?

 

[FEF]

The reason I'm sold on the 5th wheel, is the ease of connect and disconnect. All I have to do is get close. Also, come to find out, the gooseneck (vertical ball) won't have the angle needed either. The truck is designed for 30deg on each axle, making 60deg total. I can't get even close to that with a gooseneck.

I haven't done any modeling either, so I'm taking my best guess, too. What I come up with is an issue with the actual pin box. Clearly, it will need to be beefed up to handle the torque made when changing direction, and to mount the ring. However, as the pin boxes are not vertical, but rectangular, the most torque is seen at the pin box receiver.

The best I can figure is, the force the pin box receiver will be the same, as long as the ring is not mounted any longer, or lower, then the extended/drop pin box. If this statement is true, I think I'm good to go.

Man, why am I always the first to try stuff? :yikes:

 

[ntsqd]

Backing up a bit, when you say "pin box" are you referring to the structure on the trailer that descends down from the trailer frame and has the 5th wheel Kingpin on the end of it?

I know some of those are adjustable for length. Are you thinking to build a lunette based replacement for the lower section, or are you thinking to build something that attaches to the Kingpin that has a lunette attached to it?

Finally, the issue that puts the 5th wheel design out of consideration is the lack of angularity in the chassis CL plane? What if that were solved?

 

[FEF]

Backing up a bit, when you say "pin box" are you referring to the structure on the trailer that descends down from the trailer frame and has the 5th wheel Kingpin on the end of it?

I know some of those are adjustable for length. Are you thinking to build a lunette based replacement for the lower section, or are you thinking to build something that attaches to the Kingpin that has a lunette attached to it?

Finally, the issue that puts the 5th wheel design out of consideration is the lack of angularity in the chassis CL plane? What if that were solved?

The 'pin box' is the box like structure that is bolted to the trailer frame (with 4-6 bolts). It's fairly thin steel, but very stout becasue it's boxed. I've seen differnt lengths for the total vertical drop (4x4, 4x2, lift, ect), and different leading lengths that puts the trailer farther back, so the corners don't hit the truck. I'm considering making a new pin box to replace the stock configuration.

What I really want is a 5th wheel plate (receiver) that will allow a fair amount of travel on the roll axis. I've seen 'little rocker' hitches, but they don't have anywhere near the rotation I'd like to see on that axis.

Along this line of thinking, I was also considering modifying the rocker hitches, by cutting the width down, allowing more rotation on the roll axis. However, I'm not sure if I removed all of the width, I'd have the roll I'm looking for.

If a fifth wheel hitch was made, it would be good to have a pin that could be inserted for highway travel, or some kind of roll limiter. I'm assuming there's a reason why the roll is limited for highway use.

I was considering the pintle, because it's likely less 'engineering'. However, what I really want is an extreeme angle 5th wheel hitch.

Here's a pic of the tow rig.
View attachment 9962

View attachment 9963

 

[ntsqd]

An all of 10 mins high angle 5th concept:

 

[FEF]

That's the idea I had at first. Then I thought about something with longer tabs that would allow a pin to be placed in there, that would restrict rotation. Something like this hacked pic
View attachment 9965
I have no idea how nessassary roll restriction is. Is there a reason why most 5th wheel hitches don't allow the roll axis? I don't know.

The reason I gave up was becasue I didn't know how to calculate wall thichness, bolt size and grade, and other such things. I feel comfortable making a pin box with a lunette on it. I'd rather the 5th wheel, though. It's easier to hookup and such.

 

[ntsqd]

The problem with wanting to pin such a device will be the same as trying to pin those sway-bar disconnects.

I suspect that they don't build any pivot in this axis for two reasons; 1) In normal use the suspension can make up the difference, 2) There may be an issue with those tall travel trailers needing the cross wind stability. Might be why I've never seen one of those set up as a gooseneck.

You could either Engineer the design by calcing the Shear on the pin, Bearing area, Tear-out, Bending Moment, etc. or you could build it Agricultural/Oil Field & move on to the next project.

 

[FEF]

Well... We have an idea what's required, as there are pins about 1" holding the table on the truck. It's a double shear, too. But, we don't know how soft it is. If I had to guess, grade 5 might do. The bolts are big enough were I could drill holes for retaining pins.

If the pin was not stoping momentum, it didn't allow any movement, it would be easier to calculate.

Still, I'd go ahead with the prototype, if I didn't think it was going to get someone killed on the highway. With this design, I'm looking for a table (hitch) that only has pitch movement.

Or... I go back to the pintle idea, that has less room for design error.

 

[FEF]

Well....

DO you thing 1/4" wall at the shear point, with a 1" grade 5 bolt would do OK. My pocket Ref says it should be OK.

I'm getting closer to trying it. I'm nearly sure it will work, but it might be a bit wiggly at 60 mph. As the truck can't go much faster the that, I'm not worried about higher speeds.

Thanks for the help, man. Once again, You're an increadable resource.

 

[ntsqd]

One way to approximate the pin size would be to measure the minor diameter of the Kingpin. Keep in mind that you only have the one pin at this location.

1/4" wall at the pivot point seems a bit small to me, even doubled, for bearing area. Note the rings 'welded' in the model above. The idea is to produce locally sufficient thickness w/o making the thing massive.

Ideally the pin would have a surface hardness much greater than that of the plates etc. I'd consider using something like one of the HSLA steels and either case hardening it or hard chroming it. The pin should have some sort of anti-rotation feature on one of the structures. I'd put it on the upper. If there, then the lower will want grease zerk(s).

Biggest design challenge that I see is controlling the slip (slop) along the pin's axis. This is why the lower structure is laid out as individual flat plate rather than more square tube.
Probably want a thrust washer at each end of the lower structure.

Note that my model has a flat plate directly under the pin's pivot tube in the lower structure. That is intended to resist any parallelograming of the end plates.

Thank you.

 

[ntsqd]

Saw an interesting device this weekend that may have an application here.

Made by Colibert, it is an adapter that converts a RV 5th wheel to a gooseneck coupling. Their web page seems exceptionally poorly suited to gleaning info from so you may have to resort to contacting a vendor.

 

[Section 8]

Terrible idea, never mind.